The present invention relates to a light beam scanning apparatus for scanning a plurality of laser beams in an image formation apparatus such as a digital copier or a laser printer which forms a single electrostatic latent image on a single photoconductor drum by simultaneously performing scanning and exposure on the photoconductor drum by using the plurality of laser beams.
In recent years, there have been developed various digital copiers according to scanning and exposure using a laser light beam (hereafter referred to as the light beam) and an electrophotographic process.
Recently, a multi-beam digital copier is developed for increasing an image formation speed. The multi-beam system generates a plurality of light beams and simultaneously scans a plurality of lines using these light beams.
Such a multi-beam digital copier is equipped with a semiconductor laser oscillator for generating a plurality of light beams and an optical unit as a light beam scanning apparatus. The optical unit chiefly comprises a rotating polygon such as a polygon mirror, a collimator lens and an f-xcex8 lens for reflecting each light beam from the plurality of laser oscillators toward a photoconductor drum and scanning on a photoconductor drum using each light beam.
Conventionally, the optical unit of the multi-beam digital copier controls scanning direction exposure positions and passage positions of light beams. The scanning direction exposure position control relates to controlling light beam positions in a horizontal scan direction. The passage position control relates to controlling light beam positions in a vertical scan direction.
An embodiment of this technology is proposed in patent application Ser. No. 09/667,317. According to the embodiment, a pair of sensor patterns detects passage points of light beams scanning a photoconductor drum surface in the vertical scan direction in order to control light beam positions by detecting light beams with high precision in a wide range. These sensor patterns are, say, trapezoidal, arranged symmetrically at a position equivalent to a surface to be scanned, and parallel placed with a specified interval along the light beam scanning direction. A light beam passage point in the vertical scan direction is detected according to the value found by integrating a difference between outputs of the pair of sensor patterns.
The sensor is connected to a circuit for processing detected signals. The processing circuit uses a current/voltage conversion amplifier (I/V) to convert an output current detected by each sensor to voltage. A differential amplifier outputs a voltage difference. An integrator integrates this voltage difference.
Several methods are available for further increasing detection accuracy in the light beam scanning apparatus. For example, one method is to increase a gain of an I/V conversion amplifier for converting a sensor-detected current to a voltage and increase an output signal amplitude. Another method is to increase a differential amplifier gain and increase an amplitude of an output signal from the differential amplifier. Yet another method is to decrease an integrator""s integration constant to increase the integrator sensitivity. For example, the detection accuracy can be doubled by doubling the I/V conversion amplifier gain, doubling the differential amplifier gain, or halving an integration constant (RC) of the integrator.
However, these methods for increasing the detection accuracy cause the following problems.
The following describes an example of doubling the detection accuracy by halving the integrator""s integration constant RC. Since the integration constant RC is halved, the integrator sensitivity doubles. Though an output signal from the differential amplifier is unchanged, an output signal from the integrator doubles.
It is assumed that a circuit for processing output signals from the sensor comprises a single power supply (0V to 5V). A voltage approximately ranging from 1V to 4V is output from a general operational amplifier comprising the I/V conversion amplifier, the differential amplifier, or the integrator. When an output signal exceeds the maximum output voltage for such a general operational amplifier, normal operations are not ensured. The output signal remains at a power supply voltage level or a ground level. For restoring this situation to a normal state, it is necessary to turn the power off or wait for a specified time.
Namely, when an output signal from one sensor causes an integrator output to remain at the power supply voltage or at the ground level, it is impossible to correctly integrate outputs from the other sensor. This state disables detection of light beam scanning positions.
This example explains changing the integration constant. The same applies to increasing an I/V conversion amplifier gain or a differential amplifier gain. Even if a dual-supply (xc2x1supply) circuit system is used, a similar phenomenon may occur around thexc2x1 power supply level.
When an attempt is made to improve the detection accuracy, a conventional light beam scanning apparatus is subject to limitations on a sensor width, operational amplifier capabilities, and the like, This may make it possible to provide desired detection accuracy.
Accordingly, on the conventional light beam scanning apparatus, further improving the detection accuracy may significantly narrow a detection range and cannot solve chronological changes due to environmental variations and the like.
The present invention has been made in consideration of the foregoing. It is therefore an object of the present invention to provide a light beam scanning apparatus which can provide a wide range of light beam detection, improve detection accuracy, and withstand a chronological change due to environmental variations, and the like for controlling a light beam scan position.
A light beam scanning apparatus according to the present invention comprises a light emitting device for outputting a light beam; a mirror for reflecting a light beam toward a scanned face so that the light beam output from this light emitting device scans the scanned face in a horizontal scan direction; a plurality of sensors arranged on the scanned face or a position equivalent thereto for detecting a light beam scanned by the mirror and outputting an electric signal wherein the plurality of sensors comprises a pair of patterns in which one output continuously increases and the other output continuously decreases with a change of a light beam scan position in a direction orthogonal to the horizontal scan direction; a processing circuit for outputting a value obtained by integrating a difference between respective electric signals output from the pair of sensors; and a control section for controlling a passage position of a light beam output from the light emitting device on the scanned face to a specified position based on an output value from this processing circuit.
An image formation apparatus according to the present invention has an image support whose scanned face is used for scanning a light beam based on image information and forming a latent image and image formation means for forming an image formed on this image support on an image formation medium, and comprises a light emitting device for outputting a light beam; a mirror for reflecting a light beam toward a scanned face so that the light beam output from this light emitting device scans the scanned face in a horizontal scan direction; a plurality of sensors arranged on the scanned face or a position equivalent thereto for detecting a light beam scanned by the mirror and outputting an electric signal wherein the plurality of sensors comprises a pair of patterns in which one output continuously increases and the other output continuously decreases with a change of a light beam scan position in a direction orthogonal to the horizontal scan direction; a processing circuit for outputting a value obtained by integrating a difference between respective electric signals output from the pair of sensors; and a control section for controlling a passage position of a light beam output from the light emitting device on the scanned face to a specified position based on an output value from this processing circuit.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.